The movement of the monsoon trough.
Explanation
The spatial distribution of rainfall during the Southwest Monsoon season in India is primarily governed by the position and oscillation of the Monsoon Trough. This trough is a low-pressure belt extending from the Thar Desert in the northwest to the Bay of Bengal in the east, essentially representing the position of the Inter-Tropical Convergence Zone (ITCZ) over the Indian subcontinent.
Detailed Analysis:
- The movement of the monsoon trough. is Correct: The axis of the Monsoon Trough oscillates north and south during the rainy season.
- When the axis lies over the plains (normal position), widespread rainfall occurs over the northern and central parts of India.
- When the axis shifts closer to the Himalayas (a condition known as a "break" in the monsoon), rainfall ceases over the plains and becomes concentrated in the Himalayan foothills and Northeast India. Consequently, the movement of this trough is the decisive factor in the spatial distribution of rainfall.
- The intensity of the 'Loo'. is Incorrect: The 'Loo' is a strong, hot, and dry wind that blows during the summer season (May–June) over the plains of northern India. It is a pre-monsoon phenomenon and ceases with the onset of the monsoon; thus, it does not determine rainfall distribution during the monsoon season.
- The occurrence of 'Mango showers'. is Incorrect: 'Mango showers' are pre-monsoon showers occurring towards the end of the summer season, primarily in Kerala and Karnataka. They are specific local storms aiding mango ripening and are not the mechanism governing the main monsoon's spatial distribution.
- The distance from the Tropic of Cancer. is Incorrect: While the Tropic of Cancer passes through the middle of India and influences the general climate (tropical vs. subtropical), the static distance from this latitude does not determine the dynamic spatial variations of rainfall during the monsoon season.
Key Takeaway:
The oscillation of the Monsoon Trough axis determines the "active" and "break" phases of the monsoon, thereby controlling the spatial intensity and distribution of rainfall across the Indian subcontinent.